Checking front gray cast iron brake discs (wear assessment): General

The condition of the parts can make replacement necessary under two criteria, depending on the wear of perforated brake discs:

1. Cracking in the holed (perforated) friction plate is in an advanced state.
2. Brake disc minimum thickness is not attained due to wear (material erosion due to friction).

   In practice, both forms of disc erosion occur.

   Usually, it is necessary to replace the brake discs when they fall short of the brake disc minimum thickness. Only in rare cases (for long-term race driving brake loads / very high temperature fluctuations in the friction surfaces), the cracking and perforating can proceed so quickly that a premature disc change has to be performed. Both criteria governing acceptable disc condition will be described separately in the following sections.

1. Visual inspection for cracks and evaluating cracks 
   NOTE:

   • Perforation starter cracks arise through material fatigue due to frequent and extreme heat elongation fluctuations. These fluctuations in the disc temperature, which generally arise in racing, tear the perforations in a radial direction due to material fatigue (changing heat elongation conditions). For their part, these starter cracks allow a partial relaxation of the friction plate, so that the speed at which the cracks develop is very slow.
   • The maximum permissible perforation crack length is 5 mm.
   • The growth of the perforation starter cracks or a tearing of the rim of the friction plate worsens the smoothness of braking, decreases the disc strength and therefore makes a precautionary parts replacement necessary.

   Evaluating the cracks on the gray cast iron brake discs 

   • The brake discs must be replaced (as a precautionary measure), if the perforation crack length is over 5 mm (as there is no mileage reserve in the event of further extreme load) or / and the friction plate rim is torn (lessens smoothness of braking and increases the likelihood that a disc will break)!
   • In the evaluation of damage, the following drawings must be used!
   1. Corresponds to a disc exposed to above-average load.
      Fig 1: Evaluating Disc Damage Caused By Above-Average Load

      Courtesy of PORSCHE OF NORTH AMERICA, INC.
      • Need not be taken out of the vehicle!
   2. In parts, perforation starter cracks over 7 mm long. Conditions after shock braking 1,200 times (minimum desired: shock braking 200 times).
      Fig 2: Identifying Perforation Starter Cracks

      Courtesy of PORSCHE OF NORTH AMERICA, INC.
      • Brake disc can no longer be used!
   3. Brake disc with torn friction plate rim -arrow-. 
      Fig 3: Identifying Brake Disc With Torn Friction Plate Rim

      Courtesy of PORSCHE OF NORTH AMERICA, INC.
      • Brake disc can no longer be used!
2. Checking brake discs for minimum thickness 
   NOTE:

   • In the presence of high pressure forces, at the innermost and outermost hole-free friction plate track -arrows-, the brake lining friction surface is the least worn, compared to the middle holed area. Refer to Fig 4. As a result, in the case of weaker braking, a relatively higher surface pressure arises in these areas and therefore greater wear of the friction plates. The natural balance between stronger and weaker braking ensures that in almost all cases the typical wear profile of a perforated friction plate arises (inner and outer smooth friction rim area is worn the most -arrows-.
   • As opposed to smooth brake discs, by which the minimum thickness is measured in the middle of the disc (effective friction radius), in the case of perforated brake discs, the minimum thickness must be measured on the inner or outer friction surface track -arrows-, whichever is most worn.
   • Brake disc thickness/wear limit: Max. 2.0 mm total wear permissible. Refer to the technical data for the new dimension and other dimensions Refer to TECHNICAL DATA/BRAKE WEAR LIMIT - TURBO  and TECHNICAL DATA/BRAKE WEAR LIMIT - GT2 .
     Fig 4: Locating Innermost And Outermost Hole-Free Friction Plate Track

     Courtesy of PORSCHE OF NORTH AMERICA, INC.
   • Measure the smallest brake disc thickness with a suitable micrometer screw, at one of the two smooth friction rim areas -arrows-  (at the most worn friction rim area).
     Fig 5: Locating Smooth Friction Rim Areas

     Courtesy of PORSCHE OF NORTH AMERICA, INC.

   The following drawing shows the typical grooved wear profile -arrows-  of a perforated gray cast iron brake disc, which has been used to the wear limit, under tough conditions i.e. constant operation mode (test operation mode). Refer to Fig 6.

   Arrows: Zone where most wear occurs in brake disc friction surface

   Fig 6: Identifying Typical Grooved Wear Profile Of Perforated Gray Cast Iron Brake Disc

   Courtesy of PORSCHE OF NORTH AMERICA, INC.
   1. Perforation friction zone (holed friction zone)
   2. smooth friction rim zone